Controlling and/or modifying the wettability of solid surfaces has attracted significant attraction due to a wide range of potential applications, including separation, microfluidic and lab-on-a-chip devices, controllable drug delivery, self-cleaning, antibacterial and antibiofouling. For these applications, it is particularly desirable to dynamically manipulate the behaviors of the contacting liquid on the surfaces, including the contact angles, droplet motion and effective area of the solid-liquid interface.
Photoactive materials such as titanium dioxide (TiO2) have attracted significant attention due to their ability to switch wettability upon irradiation of UV light. However, the large band gap limits their ability to absorb visible light or sunlight effectively. Although doping has been demonstrated as an effective solution to enhance the visible light-induced wettability switching of photoactive materials, typically it demands harsh conditions including high temperature, high pressure or toxic chemical environments, which limits versatility in fabrication process.
Stimuli-responsive surfaces that can switch wettability towards liquids upon the application of an external stimuli have been extensively fabricated. Typically such surfaces are reversibly switched from hydrophobicity (contact angle for water, θwater >90°) or superhydrophobocity (θwater >150°) to hydrophilicity (0°<θwater<90° or superhydrophlicity (θwater=0°). In order to induce wettability switching, various external stimuli have been employed including pH, electric potential, chemical composition, solvent environment. Among numerous stimuli-responsive materials, photoactive materials, mainly of TiO2 and ZnO, are widely studied due to their ability to switch wettability from hydrophobicity to hydrophilicity upon alternation of ultraviolet (UV) irradiation and storage in dark. Recently it was revealed that hierarchical roughness on the photoactive surfaces consisting of micro- and nanostructures cause amplification of contact angle changes. Such a unique photo-induced switchable wettability of photoactive materials has attracted wide scientific attention for both fundamental research and practical applications in antibacteria, antifogging, self-cleaning, biomedical, device, fluid transportation, liquid separation, anticorrosion, offset printing, site-selective functional printing, water condensation, as well as agricultural and environmental fields.